arXiv subjects

Hiroyuki Kaneko

Publications and source records attributed to Hiroyuki Kaneko.

At least 19 recordsLinked to original sources

arXiv2026

Molecular Gas Structure and Star Formation Diversity in Stephan's Quintet Revealed by ACA CO(1-0) Mapping

We present $^{12}$CO(1-0) mapping across the entire system of Stephan's Quintet, a well-known compact galaxy group, observed by Atacama Compact Array (7\,m array + Total Power) of the Atacama Large Millimeter/submillimeter Array. These observations provide the first large-scale ($137\,\mathrm{kpc}\times119\,\mathrm{kpc}$), spatially resolved ($\sim$5.5\,$\mathrm{kpc}$) molecular gas map of a compact group. Our CO map revealed that most of the molecular gas resides in the disk of the member galaxy NGC~7319 and in the intergalactic regions, including components along the shocked filament and the optically identified tidal tail extending from NGC~7319. Along the tidal tail and its surroundings, we found not only an extended molecular gas component but also four discrete CO clumps, with velocity dispersions of $\sim$10-30 $\mathrm{km\,s^{-1}}$ and molecular gas masses of order $10^7$-$10^8\,M_\odot$. Three of these clumps spatially overlap with H\,{\sc i}, whereas the remaining clump shows no associated H\,{\sc i} or counterparts at optical and infrared wavelengths. Using star formation rates derived from H$α$ luminosities of H\,{\sc ii} regions, we found that star formation efficiencies (SFEs) span $\sim$2.2\,dex ($\sim$0.02--4\,Gyr$^{-1}$) and negatively correlate with CO velocity dispersion. While regions with small velocity dispersion exhibit SFEs comparable to those of nearby disk galaxies, those with large velocity dispersion ($\sim$50-150$\,\mathrm{km\,s^{-1}}$) around the shocked filament show strongly suppressed star formation. These results suggest that turbulence plays a significant role in regulating star formation in interacting systems.

astro-ph.GA↗

arXiv2026

Digging into the chemical complexity in the outer Galaxy: A hot molecular core in Sharpless 2-283

The outer Galaxy (galactocentric distance $\gtrsim$13.5 kpc) serves as an excellent laboratory for investigating the chemical complexity in low-metallicity environments. Here, we present the chemical analyses for the outer Galactic hot core Sh 2-283-1a SMM1 ($D_\mathrm{GC}$ = 15.7 kpc and $Z$ $\sim$0.3 $Z_\odot$), recently detected by Ikeda et al. (2025) using ALMA. Toward this source, a variety of molecular species, including complex organic molecules (COMs: CH$_3$OH, $^{13}$CH$_3$OH, CH$_2$DOH, and CH$_3$OCH$_3$) are detected. The molecular abundances relative to CH$_3$OH are similar to those of another outer Galactic hot core, demonstrating that chemically rich hot cores exist in different regions of the outer Galaxy. We also compared molecular abundances among hot cores in the inner Galaxy, outer Galaxy, and Magellanic Clouds. This comparison revealed that the metallicity-corrected $N$(SO$_2$)/$N$(H$_2$) ratios of outer Galactic hot cores are significantly lower than those of the inner Galactic ones, while their $N$(CH$_3$OH)/$N$(H$_2$) ratios are similar. The Magellanic hot cores show different trends despite having metallicities similar to those of the outer Galaxy, indicating that the chemical complexity of hot cores is governed by environmental conditions (e.g., cosmic ray intensity and dust temperature) rather than simple metallicity scaling. These environmental differences would also affect the production efficiency of COMs derived from CH$_3$OH, as the $N$(CH$_3$OCH$_3$)/$N$(CH$_3$OH) and $N$(C$_2$H$_5$OH)/$N$(CH$_3$OH) ratios in the outer Galactic sources are moderately lower than those of inner Galactic sources. The $N$(CH$_2$DOH)/$N$(CH$_3$OH) ratio of Sh 2-283-1a SMM1 is 1.5$^{+3.9}_{-1.2}$$\%$, comparable to that of inner Galactic high-mass sources.

astro-ph.GA↗

arXiv2025

CO Multi-line Imaging of Nearby Galaxies (COMING). XI. Azimuthally averaged star formation rate and stellar mass relation with molecular gas amount

This study investigated the relation between the surface density of star formation rate (SFR) ($Σ_{\mathrm{SFR}}$), stellar mass ($Σ_{M_{\ast}}$), and molecular gas mass ($Σ_{M_\mathrm{mol}}$) on nearly 1 kpc scales averaged over concentric tilted rings using the $^{12}$CO $J=1-0$ mapping data of 92 nearby galaxies obtained in the CO Multi-line Imaging of Nearby Galaxies (COMING) project. We categorized these galaxies into three groups based on the deviation of each global SFR from the star-forming main sequence (MS), $Δ$MS: upper MS (UMS), MS, and lower MS (LMS). UMS galaxies tend to be less massive or barred spiral galaxies, exhibiting molecular gas fraction ($f_{\mathrm{gas}}$) comparable to those of MS galaxies but higher star formation efficiency (SFE). In contrast, the LMS galaxies tend to be massive or active galaxies hosting an active galactic nucleus (AGN). Their $f_{\mathrm{gas}}$ values are lower than those of MS galaxies, and their SFEs are slightly lower or comparable to those of MS galaxies in the inner region. These trends indicate that enhanced SFE contributes to higher $Δ$MS values, whereas reduced $f_{\mathrm{gas}}$ results in lower $Δ$MS values. The less prominent bulge or the presence of a bar structure in UMS galaxies induces disk-wide star formation, consequently increasing the SFE. In LMS galaxies, the molecular gas is exhausted, and their star formation activity is low. Environmental effects, such as tidal gas stripping, may also reduce gas supply from the outer regions. Furthermore, our sample galaxies show that both the specific star formation rate (sSFR) and $f_{\mathrm{gas}}$ decrease in the central region in LMS galaxies but did not change in the same region in UMS galaxies. These results seem to support the inside-out quenching of star formation although the dominant cause of depletion remains uncertain.

astro-ph.GA↗

arXiv2025

Spatially and Dynamically Extended Molecular Gas in Stephan's Quintet Revealed by ALMA CO(1-0) Total Power Mapping

We present ALMA Total Power CO(1-0) mapping of Stephan's Quintet (SQ), a prototypical compact galaxy group, with a uniform noise level at a spatial scale of ~25 kpc. These observations provide the first complete view of molecular gas across the whole system. Molecular gas is found to spread over a wide area (~120 x 80 kpc), mainly over the two main member galaxies (NGC7318B and 7319), but also in the shocked ridges between these galaxies, the tidal tail, and also in intergalactic regions north of the tail. The total CO(1-0) luminosity is $(2.47\pm0.12)\times10^9~\mathrm{K~km~s^{-1}~pc^2}$, corresponding to a molecular gas mass of $(1.07\pm0.05)\times10^{10}~M_\odot$ assuming the Galactic CO-to-H2 conversion factor. The global star formation efficiency of SQ is estimated at 0.29-0.70 $\mathrm{Gyr^{-1}}$, comparable to or lower than that of nearby star-forming galaxies. Molecular gas spans a velocity range of ~1300 km/s, which can be divided into three components (low, mid, high). The low- and mid-velocity components, linked to NGC7318B and the ridge, show relatively active star formation, whereas the high-velocity component, associated with NGC7319, shows suppressed star formation. Our mapping reveals molecular gas extending ~100 kpc in projection along the inner tail and north of it, containing $(1.64\pm0.08)\times10^9~M_\odot$ (15% of total) with low velocity dispersion (~20 km/s) and ongoing star formation. While previous studies suggested in situ molecular gas formation in the tail, our data suggest an additional contribution from gas stripped from NGC7319.

astro-ph.GA↗

arXiv2025

The detection of spatially resolved protosteller outflows and episodic jets in the outer Galaxy

We present the first detection of spatially resolved protostellar outflows and jets in the outer Galaxy. We observed five star-forming regions in the outer Galaxy (Sh 2--283, NOMF05-16/19/23/63; galactocentric distance = 15.7--17.4 kpc) with the Atacama Large Millimeter/submillimeter Array (ALMA). Towards Sh 2--283, we have detected distinct outflow ($\sim$5--50 km s$^{-1}$) and jet components ($\sim$50--100 km s$^{-1}$) associated with the protostar in CO(3--2) emission. The outflows and jets are well-collimated, with the jets exhibiting multiple bullet structures. The position-velocity diagram along the CO flow axis shows two characteristic structures: (a) the flow velocity which linearly increases with the position offset from the core center (Hubble-like flow), and (b) continuous velocity components of the periodical flows (spine-like structures), which may indicate the episodic mass-ejection event. The time intervals of the mass-ejection events are estimated to be 900--4000 years based on the slopes of these spine-like structures. These characteristics align with those of nearby protostellar systems, indicating that early star formation in low-metallicity environments, such as the outer Galaxy, resembles that in the inner Galaxy. In contrast to the physical similarities, the $N\mathrm{(SiO)}$/$N\mathrm{(CO)}$ ratio in the jet bullet appears to be lower than that measured in the low-mass protostellar sources in the inner Galaxy. This may indicate the different shock chemistry or different dust composition in the outer Galaxy source, although non-LTE effects could also affect the observed low $N\mathrm{(SiO)}$/$N\mathrm{(CO)}$ ratio. We also report the new detection of the other 4 outflow sources in the outer Galaxy.

astro-ph.GA↗

arXiv2025

Infrared bubble recognition in the Milky Way and beyond using deep learning

We propose a deep learning model that can detect Spitzer bubbles accurately using two-wavelength near-infrared data acquired by the Spitzer Space Telescope and JWST. The model is based on the Single Shot MultiBox Detector as an object detection model, trained and validated using Spitzer bubbles identified by the Milky Way Project (MWP-Bubble). We found that using only MWP-Bubbles with clear structures, along with normalization and data augmentation, significantly improved performance. To reduce the dataset bias, we also use the data without bubbles in the dataset selected by combining two techniques: negative sampling and clustering. The model was optimized by hyperparameter tuning using Bayesian optimization. Applying this model to a test region of the Galactic plane resulted in a 98 $\%$ detection rate for MWP-Bubbles with 8 $μ$ m emission clearly encompassing 24 $μ$ m emission. Additionally, we applied the model to a broader area of $1^\circ \leq |l| \leq 65^\circ$, $|b| \leq 1^\circ$, including both training and validation regions, and the model detected 3,006 bubbles, of which 1,413 were newly detected. We also attempted to detect bubbles in the high-mass star-forming region Cygnus $X$, as well as in the external galaxies Large Magellanic Cloud (LMC) and NGC 628. The model successfully detected Spitzer bubbles in these external galaxies, though it also detected Mira-type variable stars and other compact sources that can be difficult to distinguish from Spitzer bubbles. The detection process takes only a few hours, demonstrating the efficiency in detecting bubble structures. Furthermore, the method used for detecting Spitzer bubbles was applied to detect shell-like structures observable only in the 8 $μ$ m emission band, leading to the detection of 469 shell-like structures in the LMC and 143 in NGC 628.

astro-ph.GA↗

arXiv2024

ALMA reveals spatially-resolved properties of molecular gas in the host galaxy of FRB 20191001A at z = 0.2340

We report the detection of the CO(2-1) emission line with a spatial resolution of 0.9 arcsec ($3.5 \mathrm{kpc}$) from the host galaxy of the fast radio burst (FRB), FRB 20191001A at $z=0.2340$, using the Atacama Large Millimeter/submillimeter Array. This is the first detection of spatially resolved CO emission from the host galaxy of an FRB at a cosmological distance. The inferred molecular gas mass of the host galaxy is $(2.3\pm0.4)\times10^{10} \mathrm{M_\odot}$, indicating that it is gas-rich, as evidenced by the measured molecular gas fraction $μ_\mathrm{gas}=0.50\pm0.22$. This molecular-gas mass and the star formation rate of the host, $\mathrm{SFR}=8.06\pm2.42 \mathrm{M_\odot yr^{-1}}$, differ from those observed in the other FRB host galaxies with the average $M_\mathrm{gas}=9.6\times10^8 \mathrm{M_\odot}$ and $\mathrm{SFR}=0.90 \mathrm{M_\odot yr^{-1}}$. This lends further credibility to the hypothesis that FRBs may originate from single or multiple progenitors across a diverse range of galaxy environments. Based on the observed velocity field modeling, we find that the molecular gas disk is dominated by an ordered circular rotation, despite the fact that the host galaxy has a gas-rich companion galaxy with a projected separation of $\sim 25 \mathrm{kpc}$. The formation of the FRB's progenitor might not have been triggered by this interaction. We derive the 3$σ$ upper limit of the molecular gas column density at the FRB detection site to be $< 2.1\times 10^{21} \mathrm{cm^{-2}}$ with a 3$σ$ upper limit.

astro-ph.GA↗

arXiv2024

Revisiting the Dragonfly Galaxy II. Young, radiatively efficient radio-loud AGN drives massive molecular outflow in a starburst merger at z=1.92

Radio-loud active galactic nuclei (RLAGNs) are a unique AGN population and were thought to be preferentially associated with supermassive black holes (SMBHs) at low accretion rates. They could impact the host galaxy evolution by expelling cold gas through the jet-mode feedback. In this work, we studied CO(6-5) line emission in a high-redshift radio galaxy, MRC 0152-209, at z=1.92 using ALMA up to a $0.024''$-resolution (corresponding to ~200 pc). This system is a starburst major merger constituted of two galaxies: the northwest (NW) one hosting the RLAGN with jet kinetic power $L_{\rm jet}\gtrsim2\times10^{46}$ erg/s and the southeast (SE) one. Based on the SED fitting for the entire system (NW+SE galaxies), we found AGN bolometric luminosity $L_{\rm AGN,bol}\sim(0.9-3)\times10^{46}$ erg/s for the RLAGN. We estimated BH mass through $M_{\rm BH}-M_\star$ scaling relations and found an Eddington ratio of $\sim0.7-4$ conservatively. These results suggest that the RLAGN is radiatively efficient and the powerful jets could be launched from a super-Eddington accretion disc. ALMA reveals a massive ($M_{\rm H_2}\sim2\times10^9$ Msun), compact ($\sim500$ pc), and lopsided molecular outflow perpendicular to the jet axis. The mass outflow rate (~1200-2600 Msun/yr) is comparable with the star formation rate of ~2000-3000 Msun/yr. The outflow kinetic power/$L_{\rm AGN,bol}$ ratio of ~0.008-0.02 and momentum boost factor ~3-24 agree with the radiative-mode AGN feedback. On the other hand, the jets can also drive the molecular outflow within its lifetime of $\sim2\times10^5$ yr without additional energy supply from AGN radiation. The jets then could remove all cold gas from the host galaxy through long-term, episodic launching. Our study reveals a unique object where starburst, powerful jets, and rapid BH growth co-exist, which may represent a fundamental stage of AGN-host galaxy co-evolution.

astro-ph.GA↗

arXiv2023

Volume density structure of the NGC253 CMZ through ALCHEMI excitation analysis

We present a spatially-resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the ALMA Large program ALCHEMI, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way's Galactic Center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3-7 transitions from nine molecular species to delineate the position-position-velocity distributions of column density ($N_\mathrm{H_2}$), volume density ($n_\mathrm{H_2}$), and temperature ($T_\mathrm{kin}$) at 27 pc resolution. Two distinct components are detected: a low-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim(10^{3.3}\ \mathrm{cm}^{-3}, 85 K)$ and a high-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim (10^{4.4}\ \mathrm{cm}^{-3}, 110\ \mathrm{K})$, separated at $n_\mathrm{H_2}\sim10^{3.8}\ \mathrm{cm}^{-3}$. NGC 253 has $\sim10$ times the high-density gas mass and $\sim3$ times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of $\sim30$ difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The $n_\mathrm{H_2}$ histogram toward NGC 253 exhibits a shallow declining slope up to $n_\mathrm{H_2}\sim10^6\ \mathrm{cm}^{-3}$, while that of the GC steeply drops in $n_\mathrm{H_2}\gtrsim10^{4.5}\ \mathrm{cm}^{-3}$ and vanishes at $10^5\ \mathrm{cm}^{-3}$. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold $n_\mathrm{H_2}$ for the dense gas is taken at $\sim 10^{4.2\mbox{-}4.6}\ \mathrm{cm}^{-3}$. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies.

astro-ph.GA↗

arXiv2023

Predicting reliable H$_2$ column density maps from molecular line data using machine learning

The total mass estimate of molecular clouds suffers from the uncertainty in the H$_2$-CO conversion factor, the so-called $X_{\rm CO}$ factor, which is used to convert the $^{12}$CO (1--0) integrated intensity to the H$_2$ column density. We demonstrate the machine learning's ability to predict the H$_2$ column density from the $^{12}$CO, $^{13}$CO, and C$^{18}$O (1--0) data set of four star-forming molecular clouds; Orion A, Orion B, Aquila, and M17. When the training is performed on a subset of each cloud, the overall distribution of the predicted column density is consistent with that of the Herschel column density. The total column density predicted and observed is consistent within 10\%, suggesting that the machine learning prediction provides a reasonable total mass estimate of each cloud. However, the distribution of the column density for values $> \sim 2 \times 10^{22}$ cm$^{-2}$, which corresponds to the dense gas, could not be predicted well. This indicates that molecular line observations tracing the dense gas are required for the training. We also found a significant difference between the predicted and observed column density when we created the model after training the data on different clouds. This highlights the presence of different $X_{\rm CO}$ factors between the clouds, and further training in various clouds is required to correct for these variations. We also demonstrated that this method could predict the column density toward the area not observed by Herschel if the molecular line and column density maps are available for the small portion, and the molecular line data are available for the larger areas.

astro-ph.GA↗

arXiv2023

Distributions of the Density and Kinetic Temperature of the Molecular Gas in the Central Region of NGC 613 using Hierarchical Bayesian Inference

We present position-position-velocity (PPV) cubes of the physical and chemical properties of the molecular medium in the central 1.2 kpc region of the active galaxy NGC 613 at a PPV resolution of 0.$^{\prime\prime}$8$\times$0.$^{\prime\prime}$8$\times$10 km s$^{-1}$ (0.$^{\prime\prime}$8 = $\sim$68 pc). We used eight molecular lines obtained with ALMA. Non-LTE calculation with hierarchical Bayesian inference was used to construct PPV cubes of the gas kinetic temperature ($T_\mathrm{kin}$), molecular hydrogen volume density ($n_\mathrm{H_2}$), column densities ($N_\mathrm{H_2}$), and fractional abundances of four molecules ($^{12}$C$^{18}$O, HCN, HCO$^+$, and CS). The derived $n_\mathrm{H_2}$, $N_\mathrm{H_2}$, and $T_\mathrm{kin}$ ranged 10$^{3.21-3.85}$ cm$^{-3}$, 10$^{20.8-22.1}$ cm$^{-2}$, and 10$^{2.33-2.64}$ K, respectively. Our first application of the non-LTE method with the hierarchical Bayesian inference to external galaxies yielded compatible results compared with the previous studies of this galaxy, demonstrating the efficacy of this method for application to other galaxies. We examined the correlation between gas surface density $Σ_\mathrm{H_2}$ (converted from $N_\mathrm{H_2}$) and the star formation rate $Σ_\mathrm{SFR}$ obtained from the 110 GHz continuum flux map and found two distinct sequences in the $Σ_\mathrm{H_2}$-$Σ_\mathrm{SFR}$ diagram; the southwestern subregion of the star-forming ring exhibited a $\sim$0.5 dex higher star formation efficiency (SFE; $Σ_\mathrm{SFR}/Σ_\mathrm{H_2}$) than the eastern subregion. However, they exhibited no systematic difference in $n_\mathrm{H_2}$, which is often argued as a driver of SFE variation. We suggest that the deficiency of molecular gas in the southwestern subregion, where no significant gas supply is evident along the offset ridges in the bar, is responsible for the elevated SFE.

astro-ph.GA↗

arXiv2023

Nobeyama 45-m CO J=1-0 Observations of Luminous Type 1 AGNs at $z\approx0.3$

We have performed CO J =1-0 observations of ten galaxies hosting luminous ($L_{\rm bol} > 10^{46}\,{\rm erg\,s^{-1}}$) type 1 active galactic nuclei (AGNs) with the Nobeyama 45-m radio telescope. The targets are selected because they are expected to be rich in molecular gas based on their high nebular dust extinction ($A_{\rm V}$). However, no significant CO emission lines were detected in any of the targets. The upper limits of the CO J=1-0 luminosities are lower than expected given the molecular gas mass inferred from the nebular $A_{\rm V}$. This inconsistency may be due to overestimated $A_{\rm V}$ values due to the lack of stellar absorption correction. Considering more reliable $A_{\rm V}$ values, the CO J=1-0 non-detections by Nobeyama 45-m are natural. This suggests that our results do not contradict the conversion methods from $A_{\rm V}$ to molecular gas mass proposed in the literature. This survey suggests that careful $A_{\rm V}$ measurements as well as CO observations are still needed to improve the measurements or estimates of the molecular gas content of galaxies hosting luminous AGNs.

astro-ph.GA↗

arXiv2023

CO Multi-line Imaging of Nearby Galaxies (COMING). XII. CO-to-H$_{2}$ Conversion Factor and Dust-to-Gas Ratio

We simultaneously measured the spatially-resolved CO-to-H$_{2}$ conversion factor ($α_\mathrm{CO}$) and dust-to-gas ratio (DGR) in nearby galaxies on a kiloparsec scale. In this study, we used $^{12}$CO($J=1-0$) data obtained by the Nobeyama 45-m radio telescope with HI and dust mass surface densities. We obtained the values of global $α_\mathrm{CO}$ and DGR in 22 nearby spiral galaxies, with averages of $2.66 \pm 1.36\ M_\odot\ \mathrm{pc}^{-2}\ (\mathrm{K\ km\ s^{-1}})^{-1}$ and $0.0052 \pm 0.0026$, respectively. Furthermore, the radial variations of $α_\mathrm{CO}$ and DGR in four barred spiral galaxies (IC 342, NGC 3627, NGC 5236, and NGC 6946) were obtained by dividing them into the inner and outer regions with a boundary of $0.2R_{25}$, where $R_{25}$ is the isophotal radius at 25 mag arcsec$^{-2}$ in the $B$ band. The averages of $α_\mathrm{CO}$ and DGR in the inner region ($\leq 0.2R_{25}$) are $0.36 \pm 0.08\ M_\odot\ \mathrm{pc}^{-2}\ (\mathrm{K\ km\ s^{-1}})^{-1}$ and $0.0199 \pm 0.0058$, while those in the outer region ($> 0.2R_{25}$) are $1.49 \pm 0.76\ M_\odot\ \mathrm{pc}^{-2}\ (\mathrm{K\ km\ s^{-1}})^{-1}$ and $0.0084 \pm 0.0037$, respectively. The value of $α_\mathrm{CO}$ in the outer region is 2.3 to 5.3 times larger than that of the inner region. When separated into the inner and outer regions, we find that $α_\mathrm{CO}$ and DGR correlate with the metallicity and the star formation rate surface density. The value of $α_\mathrm{CO}$ derived in this study tends to be smaller than those obtained in previous studies for the Milky Way and nearby star-forming galaxies. This fact can be attributed to our measurements being biased toward the inner region; we measured $α_\mathrm{CO}$ at 0.85 and 0.76 times smaller in radius than the previous works for nearby star-forming galaxies and the Milky Way, respectively.

astro-ph.GA↗

arXiv2023

Revisiting the Dragonfly Galaxy I. High-resolution ALMA and VLA Observations of the Radio Hotspots in a Hyper-luminous Infrared Galaxy at $z=1.92$

Radio-loud active galactic nuclei (RLAGNs) are rare among AGN populations. Lacking high-resolution and high-frequency observations, their structure and evolution stages are not well understood at high redshifts. In this work, we report ALMA 237 GHz continuum observation at $0.023''$ resolution and VLA 44 GHz continuum observation at $0.08''$ resolution of the radio continuum emission from a high-redshift radio and hyper-luminous infrared galaxy at $z=1.92$. The new observations confirm the South-East (SE) and North-West (NW) hotspots identified by previous low-resolution VLA observations at 4.7 and 8.2 GHz and identify a radio core undetected in all previous observations. The SE hotspot has a higher flux density than the NW one does by a factor of 6, suggesting that there can be a Doppler boosting effect in the SE one. In this scenario, we estimate the advance speed of the jet head, ranging from $\sim$0.1c -- 0.3c, which yields a mildly relativistic case. The projected linear distance between the two hotspots is $\sim13$ kpc, yielding a linear size ($\leq20$ kpc) of a Compact-Steep-Spectrum (CSS) source. Combined with new \black{high-frequency ($ν_\text{obs}\geq44$ GHz) and archived low-frequency observations ($ν_\text{obs}\leq8.2$ GHz)}, we find that injection spectra of both NW and SE hotspots can be fitted with a continuous injection (CI) model. Based on the CI model, the synchrotron ages of NW and SE hotspots have an order of $10^5$ yr, consistent with the order of magnitude $10^3 - 10^5$ yr observed in CSS sources associated with radio AGNs at an early evolution stage. The CI model also favors the scenario in which the double hotspots have experienced a quiescent phase, suggesting that this RLAGN may have transient or intermittent activities.

astro-ph.GA↗

arXiv2023

Linearly-involved Moreau-Enhanced-over-Subspace Model: Debiased Sparse Modeling and Stable Outlier-Robust Regression

We present an efficient mathematical framework based on the linearly-involved Moreau-enhanced-over-subspace (LiMES) model. Two concrete applications are considered: sparse modeling and robust regression. The popular minimax concave (MC) penalty for sparse modeling subtracts, from the $\ell_1$ norm, its Moreau envelope, inducing nearly unbiased estimates and thus yielding remarkable performance enhancements. To extend it to underdetermined linear systems, we propose the projective minimax concave penalty using the projection onto the input subspace, where the Moreau-enhancement effect is restricted to the subspace for preserving the overall convexity. We also present a novel concept of stable outlier-robust regression which distinguishes noise and outlier explicitly. The LiMES model encompasses those two specific examples as well as two other applications: stable principal component pursuit and robust classification. The LiMES function involved in the model is an ``additively nonseparable'' weakly convex function but is defined with the Moreau envelope returning the minimum of a ``separable'' convex function. This mixed nature of separability and nonseparability allows an application of the LiMES model to the underdetermined case with an efficient algorithmic implementation. Two linear/affine operators play key roles in the model: one corresponds to the projection mentioned above and the other takes care of robust regression/classification. A necessary and sufficient condition for convexity of the smooth part of the objective function is studied. Numerical examples show the efficacy of LiMES in applications to sparse modeling and robust regression.

eess.SP↗

arXiv2023

Investigating physical states of molecular gas in the overlapping region of interacting galaxies NGC4567/4568 using ALMA

We present ALMA observations of a diffuse gas tracer, CO(J = 1-0), and a warmer and denser gas tracer, CO(J = 3-2), in the overlapping region of interacting galaxies NGC 4567/4568, which are in the early stage of interaction. To comprehend the impact of galaxy interactions on molecular gas properties, we focus on interacting galaxies during the early stage and study their molecular gas properties. In this study, we investigate the physical states of a filamentary molecular structure at the overlapping region, which was previously reported. Utilising new higher-resolution CO(J = 1-0) data, we identify molecular clouds within overlapping and disc regions. Although the molecular clouds in the filament have a factor of two higher an average virial parameter (0.56+-0.14) than that in the overlapping region (0.28+-0.12) and in the disc region (0.26+-0.16), all identified molecular clouds are gravitationally bound. These clouds in the filament also have a larger velocity dispersion than that in the overlapping region, suggesting that molecular gas and/or atomic gas with different velocities collide there. We calculate the ratio of the integrated intensity of CO(J = 3-2) and CO(J = 1-0) (= R3-2/1-0) on the molecular cloud scale. The maximum R3-2/1-0 is 0.17+-0.04 for all identified clouds. The R3-2/1-0 of the molecular clouds in the filament is lower than that of the surrounding area. This result contradicts the predictions of previous numerical simulations, which suggested that the molecular gas on the collision front of galaxies is compressed and becomes denser. Our results imply that NGC 4567/4568 is in a very early stage of interaction; otherwise, the molecular clouds in the filament would not yet fulfil the conditions necessary to trigger star formation.

astro-ph.GA↗

arXiv2022

Distance determination of molecular clouds in the 1st quadrant of the Galactic plane using deep learning : I. Method and Results

Machine learning has been successfully applied in varied field but whether it is a viable tool for determining the distance to molecular clouds in the Galaxy is an open question. In the Galaxy, the kinematic distance is commonly employed as the distance to a molecular cloud. However, there is a problem in that for the inner Galaxy, two different solutions, the ``Near'' solution, and the ``Far'' solution, can be derived simultaneously. We attempted to construct a two-class (``Near'' or ``Far'') inference model using a Convolutional Neural Network (CNN), a form of deep learning that can capture spatial features generally. In this study, we used the CO dataset toward the 1st quadrant of the Galactic plane obtained with the Nobeyama 45-m radio telescope (l = 62-10 degree, |b| < 1 degree). In the model, we applied the three-dimensional distribution (position-position-velocity) of the 12CO (J=1-0) emissions as the main input. The dataset with ``Near'' or ``Far'' annotation was made from the HII region catalog of the infrared astronomy satellite WISE to train the model. As a result, we could construct a CNN model with a 76% accuracy rate on the training dataset. By using the model, we determined the distance to molecular clouds identified by the CLUMPFIND algorithm. We found that the mass of the molecular clouds with a distance of < 8.15 kpc identified in the 12CO data follows a power-law distribution with an index of about -2.3 in the mass range of M >10^3 Msun. Also, the detailed molecular gas distribution of the Galaxy as seen from the Galactic North pole was determined.

astro-ph.GA↗

arXiv2022

Multi-wavelength and Multi-CO View of The Minor Merger Driven Star Formation in the Nearby LIRG NGC 3110

We present Atacama Large Millimeter/submillimeter Array observations of multiple CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) lines and 2.9 mm and 1.3 mm continuum emission toward the nearby interacting luminous infrared galaxy NGC 3110, supplemented with similar spatial resolution H$α$, 1.4GHz continuum, and $K$-band data. We estimate the typical CO-to-H$_2$ conversion factor of 1.7 $M_{\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ within the disk using LTE-based and dust-based H$_2$ column densities, and measure the 1-kpc scale surface densities of star formation rate ($Σ_{\rm SFR}$), super star clusters ($Σ_{\rm SSC}$), molecular gas mass, and star formation efficiency (SFE) toward the entire gas disk. These parameters show a peak at the southern part of the southern spiral arm (SFE $\sim$ 10$^{-8.2}$ yr$^{-1}$, $Σ_{\rm SFR}$ $\sim$ 10$^{-0.6}$ $M_{\odot}$ kpc$^{-2}$ yr$^{-1}$, $Σ_{\rm SSC}$ $\sim$ 6.0 kpc$^{-2}$), which is likely attributed to the on-going tidal interaction with the companion galaxy MCG-01-26-013, as well as toward the circumnuclear region. We also find that thermal free-free emission contributes to a significant fraction of the millimeter continuum emission at the southern peak position. Those measurements imply that the peak of the southern arm is an active and young star-forming region, whereas the central part of NGC 3110 is a site of long-continued star formation. We suggest that, during the early stage of the galaxy-galaxy interaction with large mass ratio that in NGC 3110, fragmentation along the main galaxy's arms is an important driver of merger-induced star formation and massive gas inflow results in dusty nuclear starbursts.

astro-ph.GA↗